One genome at a time can be exciting, but two even more so. I’m not entirely sure why this is, although it may explain our fascination with sex. And what if more than two entities were involved? What if the intimacy were not just between two individual organisms, but between a greater number of different ones? Not merely a happy couple, not even limited to a ménage à trois, but an exuberant symbiotic orgy? What could be lovelier?

High on the list of symbiotic wonders are the sap-sucking insects (aphids, mealybugs, psyllids) and their bacterial endosymbionts. The partners appear to have evolved together and to have established a harmonious coexistence eons ago (see Fig. 1). This follows from the fact that plant sap is essentially a sugar solution, so if that’s all an insect eats, how does it get the amino acids, vitamins, and other essential nutrients it cannot make itself? The answer is from the endosymbiotic bacteria it carries within specialized cells in its abdomen called bacteriocytes. These welcome guests provide their host with many—but not always all—the missing nutrients. This theme of “feeding through symbiosis” is repeated up the evolutionary scale, culminating in our phylogenetic neighborhood with the ruminants.